1. Field of the Invention
This invention relates to coating compositions which are especially useful for clear coating over a colored base coat.
2. Description of Related Art
U.S. Pat. No. 5,279,862 discloses a clear coating composition which can be used as the clear coat of a motorized vehicle clear coat/color coat finish. The clear coating composition has a film-forming binder and volitile organic liquid carrier, the binder including hydroxy functional acrylic polymer and organic polyisocyanate and is characterized by rapid curing to form a finish that is tack free and can be buffed as soon as 3–4 hours after application, resulting in increased productivity of the paint shop.
Even more rapid curing of the clear coat is desired for further increase in paint shop productivity. A catalyst for the hydroxy-isocyanate crosslinking reaction is present in the clear coating composition, typically an organo tin compound. When the amount of catalyst is increased to speed-up the cure, other problems arise, including decreased potlife and reduced coating quality. In the latter case, the faster cure entraps liquid carrier within the dried clear coating, causing the coating to have poor gloss and distinctness of image.
There are other indicia of increased productivity, e.g. how soon after application the film coating dries sufficiently to be dust-free, so that the painted article (vehicle) can be moved from the paint booth, to make room for the next vehicle to be painted. The vehicle can be moved outside the paint shop, i.e. into the open air, only after the film coating has dried further so as to be free of water spotting damage.